Osteoporosis is a skeletal disease characterized by bone resorption in excess of bone formation. The osteoclast is the sole bone resorbing cell and therefore provides an important avenue for novel therapeutic development. The mature osteoclast is derived from the monocyte/macrophage lineage and requires proper differentiation and cell polarization for its function, both of which are downstream of signaling through the cytokine receptor activator of NF-kB ligand (RANKL). One potential mediator of RANKL-induced osteoclast function is the atypical protein kinase C (aPKC) sub-family, composed of PKC; and PKC6. The role of these molecules in the osteoclast has not been elucidated, but preliminary data suggests that they may be important for osteoclast function. Specifically, our lab has found that RANKL can induce phosphorylation of the aPKCs and their association with an active polarization complex composed of Par-3, Par-6, and the aPKCs. It is also possible that these molecules may be important for the differentiation of the osteoclast. Therefore, we propose to study the role of the aPKCs in osteoclast differentiation and function (Specific Aim #1) by utilizing several mouse strains lacking the aPKCs alone or in combination. We will determine the effect of aPKC deletion on osteoclastogenesis in vitro (Sub-aim A) and on osteoclast function (Sub-aim B) using several different parameters (actin ring visualization, cathepsin K localization, media CTx release, and bone pit formation). Then, we will generate mutants of the aPKCs which we will retrovirally transduce into osteoclasts lacking the appropriate gene, allowing us to perform structure/function analysis of these molecules(Sub-aim C). Finally, we will examine the in vivo phenotype of mice lacking the aPKCs (using histomorphometry and micro-CT) to determine their role in skeletal homeostasis or pathological bone loss (Specific Aim #2). Overall, we will determine the role of the aPKCs in osteoclast differentiation and function in vitro and in vivo.